Dynamic spine stabilizer

ABSTRACT

A dynamic spine stabilization device is provided that includes at least one force imparting member, e.g., a spring. The force imparting member is adapted to deliver a force of between about 150 lb/inch and 450 lbs/inch, and restrict the relative travel distance between said first and second pedicles to a distance of between about 1.5 mm and 5 mm. The spinal stabilization devices also have a minimal impact on the location of the center of rotation for the spinal segment being treated. By providing resistance in the noted range and restricting the travel distance to the noted range, it has been found that the stabilization device provides a desired level of stabilization, as reflected by range of motion values that closely approximate pre-injury range of motion levels. In addition, the resistance levels are not so high as to alter the location of the center of rotation of the treated spinal segment from its normal anatomical location to levels previously obtained, thereby permitting substantially unimpeded angular motion despite the posterior presence of a stabilization device.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of two provisional patentapplications: U.S. Ser. No. 60/506,724, entitled “DYNAMIC SPINESTABILIZER”, filed Sep. 30, 2003, and U.S. Ser. No. 60/467,414, entitled“DYNAMIC SPINE STABILIZER”, filed May 2, 2003. This application is alsoa continuation-in-part application that claims the benefit of aco-pending, non-provisional patent application: U.S. Ser. No.10/835,109, entitled “DYNAMIC SPINE STABILIZER”, filed Apr. 30, 2004.The contents of each of the foregoing applications is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and apparatus for spinalstabilization. More particularly, the invention relates to a spinalstabilization device, system and/or apparatus (and associated methods)that deliver desirable levels of stabilization to a spine whilemaintaining or preserving physiologically desirable levels and/ordegrees of spinal motion.

2. Description of the Prior Art

Low back pain is one of the most expensive diseases afflictingindustrialized societies. With the exception of the common cold, itaccounts for more doctor visits than any other ailment. The spectrum oflow back pain is wide, ranging from periods of intense disabling painwhich resolve, to varying degrees of chronic pain. The conservativetreatments available for lower back pain include: cold packs, physicaltherapy, narcotics, steroids and chiropractic maneuvers. Once a patienthas exhausted all conservative therapy, the surgical options range frommicro discectomy, a relatively minor procedure to relieve pressure onthe nerve root and spinal cord, to fusion, which takes away spinalmotion at the level of pain.

Each year, over 200,000 patients undergo lumbar fusion surgery in theUnited States. While fusion is effective about seventy percent of thetime, there are consequences even to these successful procedures,including a reduced range of motion and an increased load transfer toadjacent levels of the spine, which accelerates degeneration at thoselevels. Further, a significant number of back-pain patients, estimatedto exceed seven million in the U.S., simply endure chronic low-backpain, rather than risk procedures that may not be appropriate oreffective in alleviating their symptoms.

New treatment modalities, collectively called motion preservationdevices, are currently being developed to address these limitations.Some promising therapies are in the form of nucleus, disc or facetreplacements. Other motion preservation devices provide dynamic internalstabilization of the injured and/or degenerated spine, without removingany spinal tissues. A major goal of this concept is the stabilization ofthe spine to prevent pain while preserving near normal spinal function.The primary difference in the two types of motion preservation devicesis that replacement devices are utilized with the goal of replacingdegenerated anatomical structures which facilitates motion while dynamicinternal stabilization devices are utilized with the goal of stabilizingand controlling abnormal spinal motion without removing any tissue.

Over ten years ago a hypothesis of low back pain was presented in whichthe spinal system was conceptualized as consisting of the spinal column(vertebrae, discs and ligaments), the muscles surrounding the spinalcolumn, and a neuromuscular control unit which helps stabilize the spineduring various activities of daily living. Panjabi M M. “The stabilizingsystem of the spine. Part I. Function, dysfunction, adaptation, andenhancement.” J Spinal Disord 5 (4): 383-389, 1992a. A corollary of thishypothesis was that strong spinal muscles are needed when a spine isinjured or degenerated. This was especially true while standing inneutral posture. Panjabi M M. “The stabilizing system of the spine. PartII. Neutral zone and instability hypothesis.” J Spinal Disord 5 (4):390-397, 1992b. In other words, a low-back patient needs to havesufficient well-coordinated muscle forces, strengthening and trainingthe muscles where necessary, so they provide maximum protection whilestanding in neutral posture.

Dynamic stabilization (non-fusion) devices need certain functionality inorder to assist the compromised (injured or degenerated with diminishedmechanical integrity) spine of a back patient. Specifically, the devicesmust provide mechanical assistance to the compromised spine, especiallyin the neutral zone where it is needed most. The “neutral zone” refersto a region of low spinal stiffness or the toe-region of theMoment-Rotation curve of the spinal segment (see FIG. 1). Panjabi M M,Goel V K, Takata K. 1981 Volvo Award in Biomechanics. “PhysiologicalStrains in Lumbar Spinal Ligaments, an in vitro Biomechanical Study.”Spine 7 (3): 192-203, 1982. The neutral zone is commonly defined as thecentral part of the range of motion around the neutral posture where thesoft tissues of the spine and the facet joints provide least resistanceto spinal motion. This concept is nicely visualized on aload-displacement or moment-rotation curve of an intact and injuredspine as shown in FIG. 1. Notice that the curves are non-linear; thatis, the spine mechanical properties change with the amount ofangulations and/or rotation. If we consider curves on the positive andnegative sides to represent spinal behavior in flexion and extensionrespectively, then the slope of the curve at each point representsspinal stiffness. As seen in FIG. 1, the neutral zone is the lowstiffness region of the range of motion.

Experiments have shown that after an injury of the spinal column or dueto degeneration, neutral zones, as well as ranges of motion, increase(see FIG. 1). However, the neutral zone increases to a greater extentthan does the range of motion, when described as a percentage of thecorresponding intact values. This implies that the neutral zone is abetter measure of spinal injury and instability than the range ofmotion. Clinical studies have also found that the range of motionincrease does not correlate well with low back pain. Therefore, theunstable spine needs to be stabilized especially in the neutral zone.Dynamic internal stabilization devices must be flexible so as to movewith the spine, thus allowing the disc, the facet joints, and theligaments normal physiological motion and loads necessary formaintaining their nutritional well-being. The devices must alsoaccommodate the different physical characteristics of individualpatients and anatomies to achieve a desired posture for each individualpatient. Indeed, while providing spinal stabilization, it is highlydesirable to permit substantially unrestricted angular motion for thespine.

With the foregoing in mind, those skilled in the art will understandthat a need exists for a spinal stabilization device, system and/orapparatus which overcome the shortcomings of prior art devices. Thepresent invention provides an advantageous device, system, apparatus andassociated methods for spinal stabilization that deliver desirablelevels and/or degrees of stabilization while maintaining and/orpreserving physiologically desirable levels and/or degrees of spinalmotion.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a methodfor spinal stabilization that provides desirable levels of spinalstabilization while simultaneously permitting substantially unrestrictedangular motion of the spine. The advantageous method of the presentdisclosure is achieved by securing a dynamic stabilizer to vertebrae ofa spine and providing mechanical assistance in the form of resistance toa region of the spine to which the dynamic stabilizer is attached. In anexemplary embodiment of the present disclosure, the resistance isapplied such that greater mechanical assistance is provided while thespine is around its neutral zone and lesser mechanical assistance isprovided while the spine bends beyond its neutral zone.

According to further exemplary embodiments of the present invention, thedisclosed spinal stabilization method involves providing a spinalstabilization device that delivers a predetermined level of resistance,while accommodating a predetermined travel distance (i.e., lineartravel) between adjacent pedicles. To achieve the advantageous clinicalresults disclosed herein, the spinal stabilization device for use in thedisclosed method is adapted to provide a predetermined level ofresistance in the range of about 150 lbs/inch to about 450 lbs/inch. Inaddition, the spinal stabilization device for use in the disclosedmethod is adapted to permit a predetermined travel distance of about 1.5mm to about 5 mm.

The present invention also provides an advantageous spinal stabilizationdevice, system and/or apparatus that provides a predetermined level ofresistance while simultaneously accommodating a predetermined traveldistance (i.e., linear travel (Δx) between adjacent pedicles). Inexemplary embodiments of the present disclosure, the disclosed dynamicstabilization device, system or apparatus is adapted for posteriorplacement and is adapted to provide a predetermined level of resistancein the range of about 150 to about 450 lbs/inch, and preferably betweenabout 200 and about 400 lbs/inch, and to permit a predetermined traveldistance of about 1.5 mm and about 5 mm, and preferably between about 2mm and about 4 mm.

According to exemplary embodiments of the present disclosure, thedynamic stabilization device, system or apparatus moves under thecontrol of spinal motion, providing increased mechanical support withina central zone corresponding substantially to a neutral zone of aninjured spine. Exemplary dynamic stabilization devices include a supportassembly and a resistance assembly associated with the support assembly.The resistance assembly generates resistance, applying greaterresistance to movement during movement within the central zone and lowerresistance to movement beyond the central zone.

Other objects and advantages of the present invention will becomeapparent from the following detailed description when viewed inconjunction with the accompanying drawings, which set forth certainembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

To assist those of ordinary skill in the art in making and using thedisclosed spinal stabilization devices, systems and apparatus (and theassociated methods), reference is made to the accompanying figures,wherein:

FIG. 1 is Moment-Rotation curve for a spinal segment (intact andinjured), showing low spinal stiffness within a neutral zone.

FIG. 2 is a schematic representation of a spinal segment in conjunctionwith a Moment-Rotation curve for a spinal segment, showing low spinalstiffness within the neutral zone.

FIG. 3 a is a schematic diagram of an exemplary spinal stabilizationdevice according to the present invention in conjunction with aForce-Displacement curve, demonstrating the increased resistanceprovided within the central zone thereof.

FIG. 3 b is a Force-Displacement curve demonstrating the change inprofile achieved through replacement of springs associated with anexemplary spinal stabilization device.

FIG. 3 c is a dorsal view of the spine with a pair of spinalstabilization devices secured thereto.

FIG. 3 d is a side view showing the stabilizer in tension.

FIG. 3 e is a side view showing the stabilizer in compression.

FIG. 4 is a schematic diagram of an exemplary dynamic spinalstabilization device according to the present disclosure.

FIG. 5 is a schematic diagram of an alternate dynamic spinalstabilization device in accordance with the present disclosure.

FIG. 6 is a Moment-Rotation curve demonstrating one aspect of the mannerin which the disclosed spinal stabilization device assists spinalstabilization.

FIGS. 7 a and 7 b are respectively a free body diagram of an exemplaryspinal stabilization device according to the present disclosure, and adiagram representing a central zone of the spinal stabilization device.

FIG. 8 is a bar graph reflecting flexion/extension data based on cadaverstudies that included an exemplary dynamic spinal stabilization deviceaccording to the present disclosure.

FIG. 9 is a bar graph reflecting lateral bending data based on cadaverstudies that included an exemplary dynamic spinal stabilization deviceaccording to the present disclosure.

FIG. 10 is a bar graph reflecting axial rotation data based on cadaverstudies that included an exemplary dynamic spinal stabilization deviceaccording to the present disclosure.

FIG. 11 is a bar graph reflecting range of motion (ROM) and travel for aplurality of dynamic stabilization systems.

FIG. 12 is a plot of a range of motion (ROM) ratio versus springstiffness for dynamic stabilization systems.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary embodiments of spinal stabilization devices, systems andapparatus (and associated methods) of the present invention aredisclosed herein. It should be understood, however, that the disclosedembodiments are merely exemplary of the invention, which may be embodiedin various forms. Therefore, the details disclosed herein are not to beinterpreted as limiting, but rather as exemplary teachings that permitpersons skilled in the art to make and/or use the disclosed devices,systems and apparatus (and associated methods).

With reference to FIGS. 2, 3 a-c and 4, a method and apparatus aredisclosed for spinal stabilization. In accordance with an exemplaryembodiment of the present invention, the spinal stabilization method isachieved by securing an internal dynamic spinal stabilization device 10between adjacent vertebrae 12, 14 and thereby providing mechanicalassistance in the form of elastic resistance to the region of the spineto which the dynamic spinal stabilization device 10 is attached. Theelastic resistance is applied as a function of displacement, such thatgreater mechanical assistance is provided while the spine is in itsneutral zone and lesser mechanical assistance is provided while thespine bends beyond its neutral zone. Although the term “elasticresistance” is used throughout the body of the present specification,other forms of resistance may be employed without departing from thespirit or scope of the present invention.

As those skilled in the art will certainly appreciate, and as mentionedabove, the “neutral zone” is understood to refer to a region of lowspinal stiffness or the toe-region of the Moment-Rotation curve of thespinal segment (see FIG. 2). That is, the neutral zone may be consideredto refer to a region of laxity around the neutral resting position of aspinal segment where there is minimal resistance to inter-vertebralmotion. The range of the neutral zone is considered to be of majorsignificance in determining spinal stability. Panjabi, M M. “Thestabilizing system of the spine. Part II. Neutral zone and instabilityhypothesis.” J Spinal Disorders 1992; 5(4): 390-397.

In fact, Dr. Panjabi has previously described the load displacementcurve associated with spinal stability through the use of a “ball in abowl” analogy. According to this analogy, the shape of the bowlindicates spinal stability. A deeper bowl represents a more stablespine, while a more shallow bowl represents a less stable spine. Dr.Panjabi previously hypothesized that for someone without spinal injury,there is a normal neutral zone (that part of the range of motion wherethere is minimal resistance to inter-vertebral motion) with a normalrange of motion and, in turn, no spinal pain. In this instance, the bowlis not too deep nor too shallow. However, when an injury occurs to ananatomical structure, the neutral zone of the spinal column increasesand the ball moves freely over a larger distance. By this analogy, thebowl would be more shallow and the ball less stable and, consequently,pain results from this enlarged neutral zone.

In general, pedicle screws 16, 18 are used to attach the dynamic spinestabilization device 10 to the vertebrae 12, 14 of the spine usingwell-tolerated and familiar surgical procedures known to those skilledin the art. In accordance with a preferred embodiment, and as thoseskilled in the art will certainly appreciate, a pair of opposedstabilizers are commonly used to balance the loads applied to the spine(see FIG. 3 c). The dynamic spine stabilization device 10 assists thecompromised (injured and/or degenerated) spine of a back pain patient,and helps her/him perform daily activities. The dynamic spinestabilization device 10 does so by providing controlled resistance tospinal motion particularly around neutral posture in the region ofneutral zone. As the spine bends forward (flexion) the stabilizationdevice 10 is tensioned (see FIG. 3 d) and when the spine bends backward(extension) the stabilization device 10 is compressed (see FIG. 3 e).

The resistance to displacement provided by the dynamic spinestabilization device 10 is non-linear, being greatest in its centralzone so as to correspond to the individual's neutral zone; that is, thecentral zone of the stabilization device 10 provides a high level ofmechanical assistance in supporting the spine. As the individual movesbeyond the neutral zone, the increase in resistance decreases to a moremoderate level. As a result, the individual encounters greaterresistance to movement (or greater incremental resistance) while movingwithin the neutral zone.

According to exemplary embodiments of the present disclosure, thecentral zone of the dynamic spine stabilization device 10, that is, therange of motion in which the spine stabilization device 10 provides thegreatest resistance to movement, may be adjustable at the time ofsurgery to suit the neutral zone of each individual patient. Indeed, theresistance to movement provided by the dynamic spine stabilizationdevice 10 may be adjustable pre-operatively and intra-operatively. Thisfunctionality may serve to help to tailor the mechanical properties ofthe dynamic spine stabilization device 10 to suit the compromised spineof the individual patient. The length of the dynamic spine stabilization10 may also be adjustable intra-operatively, to suit individual patientanatomy and to achieve desired spinal posture. According to exemplaryembodiments of the present disclosure, the dynamic spine stabilizationdevice 10 can be re-adjusted post-operatively with a surgical procedureto adjust its central zone to accommodate a patient's altered needs.

In exemplary embodiments, ball joints 36, 38 link the dynamic spinestabilization device 10 with the pedicle screws 16, 18. The junction ofthe dynamic spine stabilization device 10 and pedicle screws 16, 18 insuch embodiments is free and rotationally unconstrained. Therefore,first of all, the spine is allowed all physiological motions of bendingand twisting and second, the dynamic spine stabilization device 10 andthe pedicle screws 16, 18 are protected from harmful bending andtorsional forces, or moments. While ball joints are disclosed inaccordance with an exemplary embodiment of the present invention, otherlinking structures, particularly linking structures that facilitatefreedom of relative motion between the stabilization device and thepedicle screws, may be utilized without departing from the spirit orscope of the present invention.

As there are ball joints 36, 38 at each end of the stabilization device10, bending moments are generally not transferred from the spine to thestabilization device 10. Further, the forces imparted by thestabilization device 10 with respect to the pedicle screws (andtherefore the spine) are generally those forces associated withstabilizing components and/or stabilizing assemblies/sub-assembliesassociated with stabilization device 10. As described in greater detailbelow, in an exemplary embodiment of the present disclosure, such forcesare supplied through the relative positioning, mounting and stiffness ofsprings 30, 32 which may be positioned within a housing associated withstabilization device 10. The forces imparted by the stabilization deviceare dependent upon and responsive to the tension and compression appliedto the stabilization device 10 as determined by spinal motion.Irrespective of the large loads on the spine, such as when a personcarries or lifts a heavy load, the loads impacting upon operation ofstabilization device 10 are dependent on and the result of spinalmotion, and not the result of spinal load. The stabilization device 10is, therefore, uniquely able to assist/stabilize the spine withoutenduring the high loads of the spine, allowing a wide range of designoptions pursuant to the teachings of the present disclosure.

The loading of the pedicle screws 16, 18 in exemplary implementations ofthe disclosed stabilization device 10 is also quite different from thatin prior art pedicle screw fixation systems. In general terms, the onlyload that pedicle screws 16, 18 see is the force from the stabilizationdevice 10. The forces generated by the stabilization device 10 translateinto pure axial force at the ball joint-pedicle screw interface. Thismechanism/junction arrangement greatly reduces the bending moment placedonto the pedicle screws 16, 18 as compared to prior art pedicle screwfusion systems. Thus, in exemplary embodiments of the presentdisclosure, due to the ball joints 36, 38, the bending moment within thepedicle screws 16, 18 is essentially zero at the ball joints 36, 38, andit increases toward the tip of the pedicle screws 16, 18. The area ofpedicle screw-bone interface (which can be a failure site in a typicalprior art pedicle screw fixation device) is a less stressed siterelative to prior art implementations, and is therefore not likely tofail. In sum, the pedicle screws 16, 18, when used in conjunction withspinal stabilization devices according to the present invention, carrysignificantly less load and are placed under significantly less stressthan typical pedicle screws.

In FIG. 2, the Moment-Rotation curve for a healthy spine is shown inconfigurations with the present stabilization device 10. This curveshows the low resistance to movement encountered in the neutral zone ofa healthy spine. However, when the spine is injured, this curve changesand the spine becomes unstable, as evidenced by the expansion of theneutral zone (see FIG. 1).

In accordance with an exemplary embodiment of the present invention,people suffering from spinal injuries are best treated through theapplication of increased mechanical assistance in the neutral zone. Asthe spine moves beyond the neutral zone, the necessary mechanicalassistance decreases and becomes more moderate. In particular, and withreference to FIG. 3 a, the support profile contemplated in accordancewith exemplary implementations of the present invention is disclosed.

Three different profiles are shown in FIG. 3 a. The disclosed profilesare merely exemplary and demonstrate the possible support requirementswithin the neutral zone. Profile 1 is exemplary of an individualrequiring great assistance in the neutral zone, and the central zone ofthe stabilization device is therefore increased, providing a high levelof resistance over a great displacement; Profile 2 is exemplary of anindividual where less assistance is required in the neutral zone, andthe central zone of the stabilization device is therefore more moderate,providing increased resistance over a more limited range ofdisplacement; and Profile 3 is exemplary of situations where onlyslightly greater assistance is required in the neutral zone, and thecentral zone of the stabilization device may therefore be decreased toprovide increased resistance over even a smaller range of displacement.

As those skilled in the art will certainly appreciate, the mechanicalassistance required and the range of the neutral zone will vary fromindividual to individual. However, the basic tenet of the disclosedspinal stabilization systems remains; that is, greater mechanicalassistance for those individuals suffering from spinal instability isrequired within the individual's neutral zone. This assistance isprovided in the form of greater resistance to movement provided withinthe neutral zone of the individual and the central zone of the discloseddynamic spine stabilization system 10.

The dynamic spine stabilization system 10 developed in accordance withthe present invention generally provides mechanical assistance inaccordance with the disclosed support profile. Further, in exemplaryembodiments of the present disclosure, the present stabilization device10 provides for adjustability via a concentric spring design.

More specifically, the dynamic spine stabilization system 10 providesassistance to the compromised spine in the form of increased resistanceto movement (provided by springs in accordance with a preferredembodiment) as the spine moves from the neutral posture, in anyphysiological direction. As mentioned above, the Force-Displacementrelationship provided by the dynamic spine stabilization device 10 inaccordance with the present disclosure is non-linear, with greaterincremental resistance around the neutral zone of the spine and centralzone of the stabilization device 10, and decreasing incrementalresistance beyond the central zone of the dynamic spine stabilizationdevice 10 as the individual moves beyond the neutral zone (see FIG. 3a).

The relationship of the present stabilization device 10 to forcesapplied during tension and compression is further shown with referenceto FIG. 3 a. As discussed above, the behavior of the presentstabilization device 10 is non-linear. The Load-Displacement curve hasthree zones: tension, central and compression. If K1 and K2 define thestiffness values in the tension and compression zones, respectively, thepresent stabilization device is designed such that the high stiffness inthe central zone is “K1+K2”. Depending upon the preload of thestabilization device 10, as will be discussed below in greater detail,the width of the central zone and, therefore, the region of highstiffness can be adjusted or refined.

With reference to FIG. 4, an exemplary dynamic spine stabilizationdevice 10 in accordance with the present invention is disclosed. Thedynamic spine stabilization device 10 includes a support assembly in theform of a housing 20 composed of a first housing member 22 and a secondhousing member 24. The first housing member 22 and the second housingmember 24 are telescopically connected via external threads formed uponthe open end 26 of the first housing member 22 and internal threadsformed upon the open end 28 of the second housing member 24. In thisway, the housing 20 is completed by screwing the first housing member 22into the second housing member 24. As such, and as will be discussedbelow in greater detail, the relative distance between the first housingmember 22 and the second housing member 24 can be readily adjusted forthe purpose of adjusting the compression of the first spring 30 andsecond spring 32 contained within the housing 20. Although springs areemployed in accordance with a preferred embodiment of the presentinvention, other elastic members may be employed without departing fromthe spirit or scope of the present invention. A piston assembly 34 linksthe first spring 30 and the second spring 32 to first and second balljoints 36, 38. The first and second ball joints 36, 38 are in turnshaped and designed for selective attachment to pedicle screws 16, 18extending from the respective vertebrae 12, 14.

The first ball joint 36 is secured to the closed end 39 of the firsthousing member 22 via a threaded engagement member 40 shaped anddimensioned for coupling, with threads formed within an aperture 42formed in the closed end 39 of the first housing member 22. In this way,the first ball joint 36 substantially closes off the closed end 39 ofthe first housing member 22. The length of the dynamic spinestabilization device 10 may be readily adjusted by rotating the firstball joint 36 to adjust the extent of overlap between the first housingmember 22 and the engagement member 40 of the first ball joint 36. Asthose skilled in the art will certainly appreciate, a threadedengagement between the first housing member 22 and the engagement member40 of the first ball joint 36 is disclosed in accordance with apreferred embodiment, although other coupling structures may be employedwithout departing from the spirit of the present invention.

The closed end 44 of the second housing member 24 is provided with a cap46 having an aperture 48 formed therein. As will be discussed below ingreater detail, the aperture 48 is shaped and dimensioned for thepassage of a piston rod 50 from the piston assembly 34 therethrough.

The piston assembly 34 includes a piston rod 50 and retaining rods 52that cooperate with first and second springs 30, 32. The piston rod 50includes a stop nut 54 and an enlarged head 56 at its first end 58. Theenlarged head 56 is rigidly connected to the piston rod 50 and includesguide holes 60 through which the retaining rods 52 extend duringoperation of exemplary dynamic spine stabilization device 10. As such,the enlarged head 56 is guided along the retaining rods 52 while thesecond ball joint 38 is moved toward and away from the first ball joint36. As will be discussed below in greater detail, the enlarged head 56interacts with the first spring 30 to create resistance as the dynamicspine stabilization device 10 is extended and the spine is moved inflexion.

A stop nut 54 is fit over the piston rod 50 for free movement relativethereto. However, movement of the stop nut 54 toward the first balljoint 36 is prevented by the retaining rods 52 that support the stop nut54 and prevent the stop nut 54 from moving toward the first ball joint36. As will be discussed below in greater detail, the stop nut 54interacts with the second spring 32 to create resistance as the dynamicspine stabilization device 10 is compressed and the spine is moved inextension.

The second end 62 of the piston rod 50 extends from the aperture 48 atthe closed end 44 of the second housing member 24, and is attached to anengagement member 64 of the second ball joint 38. The second end 62 ofthe piston rod 50 is coupled to the engagement member 64 of the secondball joint 38 via a threaded engagement. As those skilled in the artwill certainly appreciate, a threaded engagement between the second end62 of the piston rod 50 and the engagement member 64 of the second balljoint 38 is disclosed in accordance with a preferred embodiment,although other coupling structures may be employed without departingfrom the spirit or scope of the present invention.

As briefly mentioned above, the first and second springs 30, 32 are heldwithin the housing 20. In particular, the first spring 30 extendsbetween the enlarged head 56 of the piston rod 50 and the cap 46 of thesecond housing member 24. The second spring 32 extends between thedistal end of the engagement member 64 of the second ball joint 38 andthe stop nut 54 of the piston rod 50. The preloaded force applied by thefirst and second springs 30, 32 holds the piston rod in a staticposition within the housing, 20, such that the piston rod is able tomove during either extension or flexion of the spine.

In use, when the vertebrae 12, 14 are moved in flexion and the firstball joint 36 is drawn away from the second ball joint 38, the pistonrod 50 is pulled within the housing 24 against the force being appliedby the first spring 30. In particular, the enlarged head 56 of thepiston rod 50 is moved toward the closed end 44 of the second housingmember 24. This movement causes compression of the first spring 30,creating resistance to the movement of the spine. With regard to thesecond spring 32, the second spring 32, which is captured between stopnut 54 and second ball joint 38, extends or lengthens as a result ofmovement of second ball joint 38 away from first ball joint 36. As thevertebrae move in flexion within the neutral zone, the height (orlength) of the second spring 32 is increased, reducing the distractiveforce, and in effect increasing the resistance of the device tomovement. Through this mechanism, as the spine moves in flexion from theinitial position, both spring 30 and spring 32 resist the distraction ofthe device directly, either by increasing the load opposing the motion(i.e., first spring 30) or by decreasing the load assisting the motion(i.e., second spring 32).

However, when the spine is in extension, and the second ball joint 38 ismoved toward the first ball joint 36, the engagement member 64 of thesecond ball joint 38 moves toward the stop nut 54, which is held inplace by the retaining rods 52 as the piston rod 50 moves toward thefirst ball joint 36. This movement causes compression of the secondspring 32 held between the engagement member 64 of the second ball joint38 and the stop nut 54, to create resistance to the movement of thedynamic spine stabilization device 10. With regard to the first spring30, the first spring 30 is supported between the cap 46 and the enlargedhead 56 and, as the vertebrae move in extension within the neutral zone,the height of the second spring 30 is increased, reducing thecompressive force and, in effect, increasing the resistance of thedevice to movement. Through this mechanism, as the spine moves inextension from the initial position, both spring 32 and spring 30 resistthe compression of the device directly, either by increasing the loadopposing the motion (i.e., second spring 32) or by decreasing the loadassisting the motion (i.e., first spring 30).

Based upon the use of two concentrically positioned elastic springs 30,32 as disclosed in accordance with exemplary embodiments of the presentdisclosure, an assistance (force) profile as shown in FIG. 2 is providedby the present dynamic spine stabilization device 10. That is, the firstand second springs 30, 32 work in conjunction to provide a large elasticforce when the dynamic spine stabilization device 10 is displaced withinits central zone. However, once displacement between the first balljoint 36 and the second ball joint 38 extends beyond the central zone ofthe stabilization device 10 and the neutral zone of the individual'sspinal movement, the incremental resistance to motion is substantiallyreduced as the individual no longer requires the substantial assistanceneeded within the neutral zone. This is accomplished by setting ordefining the central zone of the stabilization device as disclosedherein. The central zone of the force displacement curve is the area ofthe curve which represents when both springs are acting in the device asdescribed above. When the motion of the spine is outside the neutralzone and the corresponding device elongation or compression is outsidethe noted central zone, the spring which is elongating reaches its freelength. Free length, as anybody skilled in the art will appreciate, isthe length of a spring when no force is applied. Thus, in exemplaryembodiments of the disclosed spinal stabilization device/mechanism, thecentral zone corresponds to a region where both springs are acting toresist motion. Outside the central zone, the resistance to movement ofthe device is only reliant on the resistance of one spring: eitherspring 30 in flexion, or spring 32 in extension.

As briefly discussed above, the dynamic spine stabilization device 10may be adjusted by rotation of the first housing member 22 relative tothe second housing member 24. This movement changes the distance betweenthe first housing member 22 and the second housing member 24 in a mannerwhich ultimately changes the preload placed across the first and secondsprings 30, 32. This change in preload alters the resistance profile ofthe present dynamic spine stabilization device 10; in cases where thedistance is reduced, the resistance profile is changed from that shownin Profile 2 of FIG. 3 a to an increase in preload (see Profile 1 ofFIG. 3 a) which enlarges the effective range in which the first andsecond springs 30, 32 act in unison. This increased width of the centralzone of the stabilizer 10 correlates to higher stiffness over a largerrange of motion of the spine. This effect can be reversed by increasingthe distance, as is evident in Profile 3 of FIG. 3 a.

Exemplary embodiments of the disclosed dynamic spine stabilizationdevice 10 are attached to pedicle screws 16, 18 extending from thevertebral section requiring support. During surgical attachment of thedynamic spine stabilization device 10, the magnitude of thestabilization device's central zone can be adjusted for each individualpatient, as judged by the surgeon and/or quantified by an instabilitymeasurement device. This adjustable feature of the disclosed dynamicspine stabilization device 10 is exemplified in the three explanatoryprofiles that have been generated in accordance with an exemplaryembodiment of the present invention (see FIG. 2; note the width of thecentral zones of the respective devices).

Pre-operatively, the first and second elastic springs 30, 32 of thedynamic spine stabilization device 10 can be replaced by a different setto accommodate a wider range of spinal instabilities. As expressed inFIG. 3 b, Profile 2 b demonstrates the force displacement curvegenerated with a stiffer set of springs when compared with the curveshown in Profile 2 a of FIG. 3 b.

Intra-operatively, the length of the dynamic spine stabilization device10 may be adjustable by turning the engagement member 40 of the firstball joint 36 to lengthen the stabilization device 10 in order toaccommodate different patient anatomies and desired spinal posture.Pre-operatively, the piston rod 50 may be replaced with a piston rod ofdiffering geometry to accommodate an even wider range of anatomicvariation.

Exemplary embodiments of the disclosed dynamic spine stabilizationdevice 10 have been tested alone to determine load-displacementrelationships. When applying tension, the dynamic spine stabilizationdevice 10 demonstrated increasing resistance up to a pre-defineddisplacement, followed by a reduced rate of increasing resistance untilthe device reached its fully elongated position. When subjected tocompression, the dynamic spine stabilization device 10 demonstratedincreasing resistance up to a pre-defined displacement, followed by areduced rate of increasing resistance until the device reached its fullycompressed position. Therefore, the dynamic spine stabilization device10 exhibits a load-displacement curve that is non-linear, with thegreatest resistance to displacement offered around the neutral posture.This behavior helps to normalize the load-displacement curve of acompromised spine.

In another exemplary embodiment of the advantageous spinal stabilizationdesigns of the present disclosure and with reference to FIG. 5, thestabilization device 110 may be constructed with an in-line springarrangement. In accordance with this embodiment, the housing 120 iscomposed of first and second housing members 122, 124 which are coupledwith threads allowing for adjustability. A first ball joint 136 extendsfrom the first housing member 122. The second housing member 124 isprovided with an aperture 148 through which the second end 162 of pistonrod 150 extends. The second end 162 of the piston rod 150 is attached tothe second ball joint 138. The second ball joint 138 is screwed onto thepiston rod 150.

The piston rod 150 includes an enlarged head 156 at its first end 158.The first and second springs 130, 132 are respectively secured betweenthe enlarged head 156 and the closed ends 139, 144 of the first andsecond housing members 122, 124. In this way, the stabilization device110 provides resistance to both expansion and compression using the samemechanical principles described for the previous exemplary embodiment.

Adjustment of the resistance profile in accordance with this alternateembodiment may be achieved by rotating the first housing member 122relative to the second housing member 124. Rotation in this way altersthe central zone of high resistance provided by the stabilization device110. As previously described, one or both springs may also be exchangedto change the slope of the force-displacement curve in two or threezones respectively.

To explain how the stabilization devices 10, 110 assist a compromisedspine (increased neutral zone), reference is made to the moment-rotationcurves of FIG. 6. Four curves are shown: 1. Intact, 2. Injured, 3.Stabilizer and, 4. Injured+Stabilizer. These are, respectively, theMoment-Rotation curves of the intact spine, injured spine, stabilizationdevice alone, and stabilization device plus injured spine. It is notedthat the fourth curve is close to the intact curve. Thus, thestabilization device, which provides greater resistance to movementaround the neutral posture, is ideally suited to compensate for theinstability of the spine.

In addition to the dynamic spine stabilizer described above, othercomplementary devices are contemplated. For example, a link-device maybe provided for joining the left- and right-stabilization units to helpprovide additional stability in axial rotation and lateral bending. Thislink-device will be a supplement to the disclosed dynamic spinestabilization devices. The link-device may be applied as needed on anindividual patient basis. In addition, a spinal stability measurementdevice may be utilized. The measurement device may be used to quantifythe stability of each spinal level at the time of surgery. The disclosedmeasurement device may be attached intra-operatively to a pair ofadjacent spinal components at compromised and uncompromised spinallevels to measure the stability of each level. The stabilitymeasurements of the adjacent uninjured levels relative to the injuredlevel(s) can be used to determine the appropriate adjustment of thedisclosed spinal stabilization device. Additionally, the stabilitymeasurements of the injured spinal level(s) can be used to adjust thedevice by referring to a tabulated database of normal, uninjured spinalstabilities. The disclosed measurement device will be simple and robust,so that the surgeon is provided with the information in the simplestpossible manner under operative conditions.

The choice of springs used in accordance with the spinal stabilizationdevices of the present invention to achieve the desired force profilecurve is generally governed, at least in part, by the basic physicallaws governing the force produced by springs. In particular, the forceprofile described above and shown in FIG. 3 a is achieved through theunique design of the present stabilization device rather than uniqueproperties of individual spring components or other elastic members.

The stabilization device of the present disclosure advantageouslyfunctions both in compression and tension, even though the two springswithin the stabilization device are both of compression type. Second,the higher stiffness (K₁+K₂) provided by the disclosed stabilizationdevice in the central zone is due to the presence of a preload. Bothsprings are made to work together when the preload is present. As thestabilization device is either tensioned or compressed within thecentral zone, the force increases in one spring and decreases in theother. When the decreasing force reaches the zero value, the springcorresponding to this force no longer functions, thus decreasing thestabilization function.

An engineering analysis, including the diagrams shown in FIGS. 7 a and 7b, is presented below. The analysis specifically relates to theexemplary embodiment disclosed in FIG. 5, although those skilled in theart will appreciate the way in which the present engineering analysisapplies to all embodiments disclosed in accordance with the presentinvention.

-   -   F₀ is the preload within the stabilization device, introduced by        shortening the body length of the housing as discussed above.    -   K₁ and K₂ are stiffness coefficients of the compression springs,        active during stabilization device tensioning and compression,        respectively.    -   F and D are, respectively, the force and displacement of the        disc of the stabilization device with respect to the body of the        stabilization device.    -   The sum of forces on the disc must equal zero. Therefore,        F+(F ₀ −D×K ₂)−(F ₀ +D×K ₁)=0, and        F=D×(K ₁ +K ₂).    -   With regard to the central zone (CZ) width (see FIG. 3 a):        -   On Tension side CZ_(T) is:            CZ _(T) =F ₀ /K ₂.        -   On Compression side CZ_(T) is:            CZ _(C) =F ₀ /K ₁.

In a broader sense, the present disclosure provides a spinalstabilization device, system and/or apparatus (and associated method(s))that deliver desirable levels of stabilization to a spine whilemaintaining or preserving physiologically desirable levels and/ordegrees of spinal motion. Thus, while providing spinal stabilization, itis also highly desirable to permit substantially unrestricted angularmotion for the spine. Indeed, a patient's unhindered ability to “bendover” with minimal effect on spinal loading despite the introduction ofa spinal stabilization device, system and/or apparatus is of primaryclinical significance.

The positioning of a spinal stabilization device posterior to the spinehas the effect of repositioning the “center of rotation” for thatsegment of the spine in a posterior direction from its normal anatomicallocation, i.e., toward the stabilization device. As used herein, theterm “center of rotation” refers to a moving point or axis around whichthe spine rotates as the spine moves in flexion and/or tension. Indeed,a non-dynamic spinal stabilization device that is positioned in aposterior direction relative to the spine, e.g., a rigid rod extendingbetween adjacent pedicle screws, will necessarily move the center ofrotation for that spinal segment in a posterior location to besubstantially coincident with the stabilization device.

Like a teeter-totter, the axis of rotation is dictated by the center ofresistive balance between the anterior and posterior anatomy of thespine. Stabilization of the spine requires imparting increasedresistance similarly to placing a large person behind a small child on ateeter-totter. Therefore, much like moving the pivot point of ateeter-totter, a spinal stabilization device is ideally designed so asto rebalance the spine. In this example, the axis of rotation of theteeter-totter would need to be moved closer to the child and largeperson to resume normal function. As the teeter-totter example clearlydemonstrates, the travel and mechanics of a dynamic system, like thespine, are significantly altered with the addition of the increasedresistance.

Posterior translation of the center of rotation is generallydisadvantageous because, as the center of rotation is moved from itsnormal anatomical location to a posterior location, the patient'sability to achieve a given level of angular motion requires a greaterdegree of travel in the region of the spine anterior to the new axis ofrotation. Stated differently, for a given amount of spinalextension/travel, a greater force will be exerted on the anterior aspectof the spine if the center of rotation has been moved to a posteriorlocation relative to its normal anatomical location. This fundamentalbiomechanical relationship is explained by the greater moment arm thatis available for angular motion when the center of rotation is at (orsubstantially near) its normal anatomical location. By moving the centerof rotation to a posterior location, e.g., by introducing a rigid spinalstabilization device to such posterior location, the moment arm issubstantially reduced, thereby restricting the availability of “normal”angular motion for the patient.

Of course, in the absence of a spinal stabilization device, the centerof rotation for a given spinal segment will remain in its normalanatomical location. However, this approach to maintaining a desiredlevel of angular motion is generally not available to a patientrequiring spinal stabilization due to injury, disease or the like. Thus,in an ideal situation, the spinal stabilization device would provide thenecessary force(s) to stabilize the spine, while simultaneouslyminimizing the degree to which the center of rotation for the treatedspinal segment is relocated from its normal anatomical location. Indeed,it is highly desirable to achieve a requisite amount or level of spinalstabilization, while having a limited or negligible impact on the centerof rotation for such spinal segment.

According to exemplary embodiments of the present disclosure, theseclinically desirable results have been found to be achieved by providinga spinal stabilization device, system or apparatus that provides apredetermined level of resistance while simultaneously accommodating apredetermined travel distance (i.e., linear travel (Δx) between adjacentpedicles), such spinal stabilization device, system or apparatus alsohaving a minimal impact on the location of the center of rotation forthe spinal segment being treated. In exemplary embodiments of thepresent disclosure, the foregoing advantageous clinical results havebeen achieved by providing a dynamic stabilization device, system orapparatus that is adapted for posterior placement, the stabilizationdevice being adapted to provide a predetermined level of resistance inthe range of about 150 to about 450 lbs/inch, and preferably betweenabout 200 and about 400 lbs/inch, and permitting a predetermined traveldistance of about 1.5 mm and about 5 mm, and preferably between about 2mm and about 4 mm.

By providing resistance in the noted range and restricting the traveldistance to the noted range, it has been found that the disclosedstabilization device provides a desired level of stabilization, asreflected by range of motion values that closely approximate pre-injuryrange of motion levels. In addition, the foregoing resistance levels arenot so high as to alter the location of the center of rotation of thetreated spinal segment from its normal anatomical location to levelspreviously obtained, thereby permitting substantially unimpeded angularmotion despite the posterior presence of a stabilization device. Thus,the disclosed dynamic spinal stabilization devices, systems andapparatus successfully address all conflicting aspects of spinalstabilization treatments, and provide advantageous clinical results thatare reflected in desired range of motion and angular motion attributes.

According to exemplary embodiments of the present disclosure, theadvantageous resistance/travel parameters set forth herein may beachieved in a variety of ways. Thus, for example, one or more springsmay be positioned with respect to a pair of pedicle screws so as toimpart the desired level of resistance, i.e., between about 150 lbs/inchand about 450 lbs/inch. The one or more springs may also be mounted,staked and/or otherwise captured with respect to the pedicle screws in amanner that limits the available travel to the desired range, i.e.,about 1.5 to about 5 mm. In an alternative implementation of the presentdisclosure, one or more non-spring elastic members may be positionedwith respect to a pair of pedicle screws so as to impart the desiredlevel of resistance, and appropriate mechanical means (e.g., one or morestops) may be associated with the spinal stabilization device, system orapparatus to limit the travel distance to the desired range. In stillfurther exemplary embodiments of the present disclosure, a plurality ofspinal stabilization systems, devices and/or apparatus are combined(e.g., in series or in parallel) to deliver the desiredresistance/travel performance parameters. Thus, for example, a firststabilization component may be provided that includes one or moresprings, and a second stabilization component that includes one or morenon-spring elastic members may be positioned in parallel (or in series)with respect to a pair of pedicle screws so as to deliver totalresistance of about 150 lbs/inch to about 450 lbs/inch and so as toaccommodate travel of about 1.5 mm to about 5 mm.

It has been found according to the present disclosure that operatingoutside the resistance and travel ranges set forth herein isdisadvantageous for purposes of spinal stabilization. More particularly,stabilization devices that impart resistance of less than about 150lbs/inch have been found to provide inadequate spinal stabilization.Conversely, stabilization devices that impart resistance of greater thanabout 450 lbs/inch have been found to provide limited incrementalstabilization effect, while undesirably increasing the rigidity of thestabilization device and moving the center of rotation in a posteriordirection from its normal anatomical location, thereby increasing theamount of anterior motion necessary to obtain said motion andunnecessarily compromising normal spinal biomechanics. In like manner,stabilization devices that limit the relative travel between adjacentpedicles, i.e., Δx, to less than about 1.5 mm to preclude desirablelevels of physiologic spinal motion, while spinal stabilization devicesthat permit relative travel between adjacent pedicles of greater thanabout 5 mm permit spinal motion that exceeds that which is necessary toprovide sufficient stabilization.

In short, spinal stabilization systems, devices and apparatus (andassociated methods) that that deliver desirable levels of stabilizationto a spine (resistance of between about 150 lbs/inch and 450 lbs/inch)while maintaining or preserving physiologically desirable levels ofspinal motion (travel of about 1.5 mm to about 5 mm) offer highlyadvantageous spinal stabilization. In addition, by providingadvantageous levels of spinal stabilization as described herein, it isfurther believed that the load experienced by pedicle screws associatedwith the disclosed spinal stabilization system, device or apparatus isreduced, thereby reducing the potential for pedicle screw failure.

Additional Experimental Results

To evaluate a stabilization device according to the present disclosure,cadaver response to applied moments in predetermined modalities wastested. In particular, measurements were made with respect to range ofmotion (ROM), neutral zone (NZ) and a high flexibility zone (HFZ). Theexperimental study was undertaken to determine whether a stabilizationdevice according to the present disclosure is effective in reducingspinal instability (measured as a reduction in NZ and HFZ), whileallowing normal ROM.

Study Design and Setting: The characteristics of five (5) cadavericmotion segments were evaluated in five (5) states: (i) intact; (ii)nucleotomy (N); (iii) nucleotomy plus stabilization device; (iv)laminectomy with partial facetectomy (LPF); and (v) LPF plusstabilization device. Each injury was chosen based on its history of useand clinical significance. Five human lumbar cadaver specimens wereused, namely four L3-4 segments and one L1-2 segment.

Methods: Specimens were obtained within 24 hours of death and stored insaline soaked gauze at −20° C. until the time of testing. The specimenswere thawed and extraneous tissue removed. Plain radiographs were takenof the spines to determine anatomy, degree of disc degeneration andpre-existing bony pathology (if any). Specimens with pathology (e.g.,bridging osteophytes, Schmol's nodes or obvious facet degeneration) wereexcluded from the study. Specimens with significant pre-existing discpathology (such as herniation) were also excluded from the study.

Pedicle screws were placed bilaterally in the inferior and superiorvertebral bodies. Additional augmentation of pedicle screw fixation wasachieved by removing the pedicle screw, adding a small amount of epoxy(≈1 cc), and reinserting the screw. Pedicle screws were wrapped insaline soaked paper and each motion segment was potted in low meltingtemperature alloy. The construct was placed in test equipment adapted toprovide multiple degrees of freedom. The potting fixture was bolted tothe testing machine such that the specimen was rigidly attached relativeto the machine. The inferior fixture rested on an x-y table whichallowed the specimen unconstrained free motion during testing.

A six-axis load cell (AMTI, Inc., Watertown, Mass.) was used to measurethe forces and torques applied to the specimen during testing. An axialcompressive load was applied continuously to the specimen (preload of200N), while pure bending moments in flexion/extension, left/rightlateral bending, and left/right torsion were applied to the superiorvertebral body of the specimen. Relative changes in position andangulation were measured with high-resolution optical encoders (GurleyPrecision Instruments, Troy, N.Y.). Displacement of the stabilizationdevice between the pedicle screws was measured using two positiontransducers (SpaceAge Control, Palmdale, Calif.). Data were collected ata minimum sampling rate of 10 Hz.

Intact specimens (no injury and no stabilization) were loaded throughthree cycles each to 10 Nm in flexion/extension, left/right lateralbending, and left/right torsion at 1 mm/minute with a continuous 200 Naxial compressive preload. Following completion of intact testing,specimens were removed from the test machine. Following placement of thestabilization device/system of the present disclosure, specimens wereplaced back into the test machine and the test protocol was repeated. Inthe tests described herein, a stabilization device of the type depictedin FIG. 5. Each motion segment was again loaded through 3 cycles offorward flexion/extension, left/right lateral bending, and left/righttorsion under a continuous compressive 200 N axial compressive pre-load.Testing was repeated under the following conditions: (i) nucleotomy withno stabilization, nucleotomy with stabilization, laminectomy withpartial facetectomy (LPF) with no stabilization, and LPF withstabilization.

Outcome Measures: Following the completion of the testing, raw data textfiles were exported to a Microsoft Excel program. Data included cyclenumber, motion, current angle, current moment, axial load, displacementtransducer on right side, and displacement transducer on left side.Range of motion at 10 Nm, neutral zone at 2.5 Nm (high flexibilityzone), neutral zone at 0.2 Nm (passive curve), and displacement of thepedicle screws of the uninstrumented constructs (i.e., in the absence ofa dynamic stabilization device according to the present disclosure) werecompared to the instrumented constructs (i.e., with a dynamicstabilization device according to the present disclosure). ROM, NZ andHFZ were reported for Flexion/Extension, Lateral Bending and AxialRotation. ROM=rotation±10N−m; NZ=rotation±0.2 Nm of the passive responseprior to crossing the zero moment axis; HFZ=rotation±2.5 Nm on theactive curve.

Results: Due to specimen degradation, two (2) specimens were notevaluated in LPF and LPF plus stabilization device. Mean range ofmotion, neutral zone and displacement data for each construct inflexion/extension, lateral bending, and axial rotation are set forth inthe bar graphs of FIGS. 8-10. As the bar graphs show, spinal instabilityincreases with surgical injury. This may be measured as an increase inROM and a significantly higher relative increase in NZ and HFZ. Throughuse of the disclosed stabilization device as described herein, it waspossible to advantageously reduce NZ and HFZ to levels that arecomparable to intact levels, while simultaneously leaving ROMuncompromised.

Further Test Results:

With reference to FIGS. 11 and 12, data supporting the criticalitydescribed herein with respect to resistance/travel parameters wasgenerated in separate testing from that described above, and such datais provided in bar chart and graphical form for two distinct specimens.With initial reference to FIG. 11, a series of spring stiffnesses weretested in the L4-L5 spinal region using a spinal stabilization deviceaccording to the present disclosure. In particular, a spinalstabilization device of the type described with reference to FIGS. 4 and5 was employed in cadaver studies to generate the data reflected inFIGS. 11 and 12. Accordingly, the spinal stabilization device includedfirst and second nested springs that were subjected to a preload of 200N. Of note, additional studies were performed with a preload of 400 Nwith consistent results. The tested spinal segment showed an intactrange of motion (ROM) of 12.44 degrees and an injured ROM (i.e., ROMpost-nucleotomy) of 13.58 degrees.

The spring stiffnesses set forth along the X-axis of the bar graphs ofFIG. 11 reflect the spring forces tested in the outer spring position.The outer spring corresponds to the “flexion” spring in the disclosedspinal stabilization device of FIGS. 4 and 5, and represents thedominant spring for purposes of characterizing the performance of thedisclosed spinal stabilization device. Experimental data has beengenerated with a relative spring stiffness of 20:10 and 10:20 betweenthe inner and outer springs, with comparable results. The data reportedherein corresponds to tests wherein the relationship between the outerspring stiffness (flexion spring) and the inner spring stiffness(tension spring) was 20:10. Thus, in the data reported on FIG. 11, threedistinct spring stiffnesses were tested for the flexion spring in anexemplary spinal stabilization device of the present disclosure, witheach spring stiffness tested in duplicate test runs. Data was collectedfor ROM (in degrees; left-most bar in each pair), and travel distance(in mm; right-most bar in each pair). Travel distance refers to thedistance that the first and second pedicle screws travel with respect toeach other and is an indicia of the degree to which the axis of rotationof the spine is effected by a spinal intervention. As the traveldistance is reduced, greater compromise of the normal motion of thespine arises.

With initial reference to the bar graphs associated with a springstiffness of 42.86 lbf/in, it is noted that the ROM exceeds the ROMassociated with an intact spine. Thus, with a spine stiffness of 42.86lbf/in for the outer spring, the disclosed spinal stabilization deviceprovides insufficient stabilization forces to reduce the ROM from theinjured level (13.58 degrees) to the intact level (12.44). Instead, theROM remains above 12.8 degrees (12.81 and 13.04 degrees), whichcorresponds to an undesirable level of spinal instability. The traveldistances associated with tests wherein the outer spring had a stiffnessof 42.86 lbf/in were 5.69 and 5.92 mm.

Turning to the middle two bar graphs, data is presented for test runsemploying an outer spring having a stiffness of 145.71 lbf/in. For thesetest runs, the ROM was advantageously reduced to a level that was belowthe intact ROM, i.e., 10.73/10.67 degrees vs. 12.44 degrees. Thisreduction in ROM reflects a desirable level of stabilization. Aconcomitant reduction in travel distance was noted relative to theweaker spring (42.86 lbf/in). More particularly, the travel distance wasreduced to 4.34/3.39 mm, reflecting an increase in the degree to which apatient's angular motion would be restricted relative to the weakerspring.

Turning to third outer spring reflected in the test data of FIG. 11, anouter spring having a stiffness of 197.14 lbf/in was tested in a spinalstabilization device of the present disclosure. Significantly, the ROMwas substantially unchanged relative to the weaker spring (145.71lbf/in), while the travel distance demonstrated further reductions(3.08/3.13 mm vs. 4.34/3.39 mm). The test data of the right-most bargraphs reflects a surprising result in spinal stabilizationapplications, namely that a threshold is reached wherein furtherincreases in spring stiffness (i.e., stabilizing force) does not effecta material reduction in ROM, while continued reductions in traveldistance are observed.

In view of the surprising results reported herein, clinicallyadvantageous spinal stabilization devices/systems according to thepresent disclosure are characterized in that they supply a stabilizingforce that substantially corresponds to the threshold level notedherein, thereby limiting the degree to which travel distance betweenadjacent pedicles is restricted/reduced. By minimizing the impact ontravel distance, spinal stabilization devices/systems of the presentdisclosure advantageously permit substantially unrestricted angularmotion of the spine, while delivering desired/necessary levels of spinalstabilization.

With reference to the graph of FIG. 12, further data supporting thesurprisingly advantageous results achieved through the disclosed spinalstabilization devices/systems is provided. The Y-axis of FIG. 12corresponds to a ratio of the ROM for an injured spine relative to anintact spine. Thus, if the injured spine was stabilized to its initialintact ROM performance, a ratio of 1.0 would be achieved. For clinicallydesirable spinal stabilization, the target ROM ratio in the testprotocols described herein is 0.8. Stated differently, a desirablespinal stabilization device/system will reduce the ROM of an injuredspine to a level that is approximately 80% of the initial intact ROMlevel.

With particular reference to FIG. 12, the initial data point (springstiffness of 0) corresponds to test data wherein the injured ROM isapproximately 10% greater than the intact ROM. Additional ROM ratio datapoints are provided for spring stiffnesses of 42.86 lbf/in, 145.71lbf/in and 197.14 lbf/in. Of note, a plateau is established at an ROMratio of about 0.82, which closely approximates the target ROM ratio of0.8. Thus, the plot of FIG. 12 further demonstrates that additionalincreases in spring stiffness beyond that necessary to achieve a ROMratio of about 0.82 is ineffective to further reduce the ROM ratio toany appreciable degree. The test results reflected in FIG. 12, andparticularly the plateau, are not predicted by a least squares fit ofthe initial data points, as reflected by the white line charted on FIG.12.

Based on the foregoing test results, it is apparent that advantageousspinal stabilization results may be achieved according to the presentdisclosure by providing spinal stabilization devices/systems thatoperate at the ROM ratio plateau described herein. It has been foundaccording to the present disclosure is achieved by impart a resistanceof about 150 lbs/inch to about 450 lbs/inch, and permitting a travel ofabout 1.5 mm to about 4.5 mm. The foregoing spinal stabilizationdevices/systems are generally effective to achieve an ROM ratio thatclosely approximates 0.8, thereby achieving advantageous levels ofstabilization while simultaneously providing substantially unrestrictedangular motion of the spine.

As those skilled in the art will certainly appreciate, the conceptsunderlying the present invention may be applied to other medicalprocedures. As such, these concepts may be utilized beyond spinaltreatments without departing from the spirit of the present invention.While preferred and exemplary embodiments have been shown and describedherein, it will be understood that there is no intent to limit theinvention by such disclosure, but rather, the present disclosure isintended to encompass all modifications and alternate constructionsfalling within the spirit and scope of the invention as defined in theappended claims. Indeed, alternative dynamic spinal stabilizationdevices for use according to the present disclosure are described in acommonly assigned U.S. patent application entitled “Systems and Methodsfor Spine Stabilization Including a Dynamic Junction,” filed on Dec. 31,2004 and assigned Ser. No. 11/027,269, the entire contents of which areincorporated herein by reference.

1. A dynamic stabilization system, comprising: a stabilizing member thatincludes at least one force imparting element, the stabilizing memberbeing adapted to be mounted with respect to first and second pedicles ofa spine, wherein the force imparting element is adapted to deliver aforce of between about 150 lb/inch and 450 lbs/inch, and restrict therelative travel distance between said first and second pedicles of saidspine to a distance of between about 1.5 mm and 5 mm.
 2. The dynamicstabilization system according to claim 1, wherein said at least oneforce imparting element is a spring.
 3. The dynamic stabilization systemaccording to claim 1, wherein said at least one force imparting elementincludes a first spring and a second spring, and wherein said first andsecond springs together deliver a force of between about 150 lb/inch and450 lbs/inch, and restrict the relative travel distance between saidfirst and second pedicles of said spine to a distance of between about1.5 mm and 5 mm.
 4. The dynamic stabilization system according to claim3, wherein said first and second springs are in a nested orientation. 5.The dynamic stabilization system according to claim 3, wherein saidfirst and second springs are in an axially aligned orientation.
 6. Thedynamic stabilization system according to claim 1, wherein saidstabilizing member includes a housing within which is positioned said atleast one force imparting element.
 7. The dynamic stabilization systemaccording to claim 6, wherein said housing includes first and secondhousing members, and wherein said first and second housing members arerepositionable with respect to each other.
 8. The dynamic stabilizationsystem according to claim 1, wherein said force stabilizing member iseffective to limit the range of motion of an injured spine toapproximately 80% of the initial uninjured spine.
 9. The dynamicstabilization system according to claim 1, wherein said stabilizingmember includes first and second mounting elements positioned atopposite ends thereof.
 10. The dynamic stabilization system according toclaim 9, wherein said first and second mounting elements are balljoints.
 11. The dynamic stabilization system according to claim 10,wherein said ball joints are adapted to be mounted with respect to firstand second pedicle screws.
 12. The dynamic stabilization systemaccording to claim 1, wherein said stabilizing member is adapted todeliver stabilizing forces to the pedicles of a spine that have alimited impact on the location of the center of rotation for said firstand second pedicles of said spine.
 13. A method for stabilizing a spinalsegment, comprising: positioning a spinal stabilization device betweenfirst and second pedicle screws that are mounted with respect to a firstand a second pedicle of a spinal segment, said spinal stabilizationdevice including at least one force imparting element that is adapted todeliver a force of between about 150 lb/inch and 450 lbs/inch, andrestrict the relative travel distance between said first and secondpedicles to a distance of between about 1.5 mm and 5 mm.
 14. The methodaccording to claim 13, wherein said at least one force imparting elementis a spring.
 15. The method according to claim 13, wherein said at leastone force imparting element includes a first and a second spring. 16.The method according to claim 13, wherein said spinal stabilizationdevice further includes first and second ends that are adapted tocooperate with pedicle mounting structures.
 17. The method according toclaim 16, wherein at least one of said pedicle mounting structures is aball joint.
 18. The method according to claim 13, wherein said at leastone force imparting element is subject to a preload.